1. Field of the Invention
This application relates broadly to devices and methods for the removal of grit, i.e., small particles like fine sand, from liquid suspensions thereof. More particularly, it concerns (a) improved methods for grit removal from water in wastewater treatment operations while mitigating inclusion of putrecibles in the removed grit and (b) improved equipment for carrying out such methods.
2. Description of the Prior Art
Sewage entering a wastewater treatment plant carries with it grit particles such as sand and other inert particles having a particle size that may vary from under 0.15 mm to over 1.00 mm, typically in a range from about 50 mesh (0.297 mm) to about 120 mesh (0.125 mm). To prevent wear on mechanical equipment within the treatment plant, the grit must be removed by grit removal devices.
There are four basic types of grit removal devices, namely, horizontal constant velocity channels, aerated grit chambers, short-term sedimentation tanks and vortex chamber grit removers (see U.S. Pat. Nos. 3,941,698 and 4,767,532). This present invention relates to the latter type of grit remover devices.
The vortex chamber devices utilize two separate forces, i.e., gravitional and centrifugal, to separate grit from the wastewater while selectively maintaining organic particles, so-called putrecibles, in suspension. Typically a submerged turbine rotor generates hydraulic profiles to provide the centrifugal forces to sweep the grit while hopefully allowing the putrecibles to remain in suspension.
The performance requirements for grit removers as stated in the WPCF 1977 edition of the Manual Of Practice On Wastewater Treatment Plant Design requires that grit of 0.2 mm (65 mesh) and larger should be removed and the discharged grit shall contain no more than 3% by weight putrecibles. Typical consulting engineer specifications require that grit removers remove 90% of grit larger than 50 mesh (0.297 mm), 80% of grit larger than 100 mesh (0.149 mm) and 60% of grit larger than 120 mesh (0.125 mm).
Grit particles vary in density so particles of the same size may exhibit differing settling velocities. Hence, in the construction and operation of vortex grit removers, primary attention has centered on overflow rate or rise rate of the wastewater passing in the vortex chamber. Thus, since settling velocities occur over a range, if the wastewater rise rate exceeds the minimum grit settling rate, loss of that size particle will result. For example, settling velocity studies have determined that 0.15-0.2 mm particles have settling rates of from about 1.68 to 8.4 ft/min and when these size particles were subject to a rise rate of 2.0 ft/min, 3.3% of particles would be lost, while 92.7% would be lost at 4.0 ft/min rise rate.
The advance in the art of construction and operation of vortex grit removers provided by the present invention results from the discovery that another factor in addition to rise rate plays an important part in the efficiency with which grit can be removed in such devices, namely, that there is a functional relationship between rise rate and the speed of rotation of the rotor imparting vortical motion to the liquid present in such chamber and that this may be used to automatically control speed of rotor rotation.